(a) Field of the Invention
The present invention relates to a transmission type of illumination system for microscopes and, more specifically to a transmission type of illumination system for microscopes which composes a complete Kohler illumination system within a wide magnification range from 1.times. to 100.times..
(b) Description of the Prior Art:
An ideal illumination system for microscopes should satisfy four requirements: (1) being free from ununiform illumination, (2) having N.A. satisfying N.A. of the objective lens to be used in combination therewith, (3) allowing field stop to be effective and (4) allowing aperture stop to be effective. Kohler illumination system is known as an illumination system satisfying these requirements. It can satisfy these requirements by so designing it as to form an image of a light source having a size large enough to satisfy N.A. of the objective lens at the position of the aperture stop, project it to the pupil plane of the objective lens and, on the other hand, form an image of field stop at the plane of object to be observed. However, since a microscope is used within a very wide magnification range from 1.times. to 100.times. between which both N.A. and size of visual field are remarkably different, it is impossible to realize an ideal Kohler illumination system at all the magnification levels with a single illumination system. For this reason, there have conventionally been invented illumination systems so adapted as to cover a wide magnification range by switching the systems at plural stages by inserting and/or removing lens(es) into and out of the optical path. Out of these illumination systems, the one disclosed by U.S. Pat. No. 3,137,761 is so adapted as to cover a magnification range from low to high levels by switching relay lenses to be used with a condenser lens employed commonly at all magnification levels. In an illumination system of this type using a condenser lens commonly at all the magnification levels, the condenser lens must have rather a long focal length for the purpose of illuminating a wide visual field at low magnification levels. However, as focal length of the condenser lens is prolonged, chromatic aberration and spherical aberration are aggravated, thereby degrading image quality undesirably at high magnification levels. Further, a prolonged focal length of the condenser lens unavoidably enlarges the illumination system as a whole. Furthermore, the illumination system of the type using the condenser lens commonly at all the magnification levels does not permit widening magnification range down to an ultra-low level, but is usable only at magnification levels down to 4.times. to 10.times..
In addition, there have conventionally been known an illumination system covering a wide magnification range which was disclosed by Japanese published unexamined patent application No. 59951/79. This illumination system composes Kohler illumination system which can cover a magnification range from 1.times. to 100.times. by switching condenser lenses and aperture stops between high and low magnification levels, and employing the relay lens system at high or low magnification level. However, this conventional examples requires a very complicated switching mechanism since the relay lens system must be inserted and removed into and out of the optical path at a position nearer the light source than the aperture stop for low magnification level is. This is due to the fact that the illumination system has an optical system folded as shown in FIG. 1 and a portion of the optical system is arranged inside a microscope stand. Further, since the relay lens R is used on the side of the light source, it is located on the optical axis inside the microscope stand. On the other hand, since the condenser lens for high magnification level H and the condenser lens for low magnification level L are located on the object side, certain optical elements must be displaced in the vertical direction indicated by the arrow A, whereas other optical elements must be displaced in the horizontal direction indicated by the arrow B shown in FIG. 1.
Though the conventional illumination systems are so designed as to be switchable at two stages of high and low magnification levels, another problem is posed when a single illumination system is used commonly in a wide magnification range. Let us assume, for example, a case where a magnification range from 10.times. to 100.times. is covered by an illumination system and a magnification range of 1.times. to 10.times. is covered by another illumination system switched from said illumination system. When an image of the light source is formed so as to satisfy N.A. of the objective lens at a magnification level of 100.times., only a portion of the light source image is usable at 10.times. at which N.A. is smaller. When N.A. is 1.3 at 100.times. and 0.25 at 10.times., for example, only 1/5 (=0.25/1.3) of the light source image is usable at 10.times.. As a result, only a portion of the filament of the illumination light source is used for illumination, thereby making illumination ununiform due to influence caused by cubic structure of the filament, etc. A simple method to prevent such ununiform illumination is to minimize the light source image to a certain degree so as not to satisfy N.A. at 100.times. for preventing ununiform illumination at 10.times., and to use a diffusion plate to cover the large N.A. at 100.times.. However, this method reduces illumination light intensity at 100.times. and makes it impossible to observe the light source image directly, thereby making optical alignment rather difficult, degrading image contrast and bringing about other various undesirable or inconvenient results.